The present invention relates to telephone sets suitable as an extension or second phone designed for direct connection to standard central station telephone lines, and having built-in logic circuitry for restricting the type and number of digits of an outbound call.
There is often a need in the home or small business for a second or extension phone that is restricted to dialing local calls. For example, in a home, family or recreation room frequented by guests of the owner an extension phone is often used by guests to make long distance toll calls that can mount up to large long distance telephone bills at the end of the month. Similarly, small businesses not equipped with sophisticated PBXs or computer control units, often have extension phones available to customers in a reception area. When such phones are connected directly to the central station telephone line or lines, unauthorized long distance calls may be placed without the consent of the business owner.
Yet, it is often desired in home and small business phone installations to provide that extra extension phone in public or quasi-public areas for enabling guests, customers, and other members of the public to place local, non-toll, outbound calls as well as allowing the owner or resident to receive all incoming calls at that particular phone extension.
While outbound call restriction systems are known in the art, for example, reference is made to U.S. Pat. No. 4,251,692 for TELEPHONE CALL RESTRICTING APPARATUS, issued to Herbert H. Waldman; U.S. Pat. No. 3,920,936 for TOLLCALL RESTRICTION SYSTEM, issued to Charles S. Mogtader; and U.S. Pat. No. 4,585,904 for PROGRAMMABLE COMPUTERIZED TELEPHONE CALL COST METERING DEVICE, issued to Mincone, et al., these and other call restricting systems are of a generally sophisticated, computerized function of a larger PBX or other local or central computer control unit that interfaces the central station lines with the individual telephone sets. Other examples of call restrictor and dialing detection circuits are set forth in U.S. Pat. No. 4,012,602 issued Mar. 15, 1977, to Joseph Monroe Jackson, and U.S. Pat. No. 4,791,667 issued Dec. 13, 1988, to inventor Carol Havel. Such PBXs and other computerized control units are cost effective for the moderate to large business installations, but are too costly and complex for the one or two line residential or small business service. Other call restrictors are of a different type of mechanical or electro-mechanical type such as exhibited by U.S. Pat. Nos. 3,874,164; 4,543,452; 1,287,099; and 2,433,366.
Further examples of restrictions on extension phones include keyed locks on the rotary dial and other types of electro-mechanical devices that can be accessed with a key or proper positioning of a control switch. These types of mechanical and electro-mechanical restrictors are often undesirable because they either make the use of the phone more difficult for the authorized person, or convey an impression of distrust to guests or customers.
It is therefore an object of the present invention to provide a modified standard telephone set of the type commonly connected directly to the tip and ring leads of the standard central station line in which a built-in call restriction logic circuit prevents all but local (non-toll) outbound calls. Furthermore, it is an object of the present invention to provide such a phone set suitable as an extension or second phone in the home or small business in which the call restriction logic circuit is capable of being built-in to the existing housing of the phone set and for this purpose the logic is designed to require only a small number of logic circuit components; is powered solely by current on the phone line supplied by central station; exhibits extraordinary reliability over a long period of use; is difficult to circumvent even by opening the housing of the phone set and has multiple call restriction modes to prevent different types of unauthorized outbound dialing.